The present invention relates to a combined light modulator device for a holographic or an autostereoscopic display with observer tracking. In the context of the present invention, a combined light modulator device is understood to be a device which changes in a multi-stage process the properties and/or the direction of light which is emitted by one or multiple real or virtual light sources.
Here, a virtual light source is a light source which is only seemingly situated at a certain position, i.e. a light source which appears to be there as a result of manipulating light of a real light source by—typically static—imaging means, such as, for example, a mirror and/or beam confining means such as apertures.
In the context of this patent application, a holographic display is a display device for three-dimensional image data where the three-dimensional object data of the scene to be represented are encoded in the form of diffraction patterns of the scene to be reconstructed. The reconstruction of a three-dimensional scene in a large visibility region at high quality requires both great computing power and a high-resolution light modulator.
In document DE 103 53 439 B4, the entire contents of which being fully incorporated herein by reference the applicant has thus proposed a method in which the wave front is only computed for a small visibility region whose diameter is only little larger than the diameter of an eye pupil of an observer eye. Consequently, each of the object points to be reconstructed only needs to be encoded in a small region of the light modulator in particular sub-holograms.
For this, sufficiently coherent light, which is emitted by at least one light source, illuminates at least one light modulator and is imaged to at least one observer eye by a field lens. The reconstruction of the three-dimensional scene for the other observer eye can be generated by alternately switching on at least one other light source in synchronism with the light modulator while writing a corresponding hologram or corresponding sub-holograms to the light modulator. Here, colour representation is possible by way of spatial or temporal interleaving (space or time division multiplexing) of the hologram information for each colour component. To enable the observer to move freely in front of the display, the focal regions are tracked to the observer eyes by switching on further light sources separately. For this, the coordinates of the eyes of one or multiple observers are continuously determined with the help of a position detection system.
Here, the reconstruction of the scene can be adapted to the new observer position by recalculating of the diffraction pattern. It is also possible to provide reconstructions for multiple observers by way of temporal interleaving (time division multiplexing).
To provide observer tracking along the optical path, the focal plane of the focusing unit, and thus the size of the visibility region, is preferably additionally adapted to the eye positions of the observers.
In an autostereoscopic display (ASD) with observer tracking, it is not diffraction patterns that are encoded on the light modulator, but rather are the scene views for the particular eye written directly.
Observer tracking can be realised by way of direct or indirect displacement of the light sources. A known example of indirect displacement are deflection mirrors.
Numerous other methods of observer tracking are known. Observer tracking can be achieved, for example, by modifying the optical path in front of or behind the light modulator which is used for hologram encoding or for stereo representation. In addition to mechanical methods, methods of changing reflective, diffractive or refractive properties using adaptive optical systems are known.
Further, it is known to use combined tracking methods, i.e. methods which take advantage of a light-deflecting function that is static but varies across the surface area of the light-deflecting means.
In the patent application DE 10 2008 054 438 A1 filed by the applicant, a matrix of electrically controllable fluid cells is proposed for observer tracking with the fluid cells having additional static light-deflecting means which, however, vary across the surface area of the matrix in order to realise or at least to support the function of a field lens. These light-deflecting means can, for example, comprise refractive elements, such as prisms or lenses, or diffractive elements, such as volume gratings or blazed gratings, i.e. gratings which are optimised for a certain wavelength.
The patent application DE 10 2009 028 626 A1 filed by the applicant, the entire contents of which being fully incorporated herein by reference, teaches to use controllable diffractive gratings for observer tracking.
Here, multiple gratings of this kind with the same direction of deflection can also be arranged one after another in order to realise a larger deflection angle. Here, it is also possible to arrange at least two controllable deflection gratings one after another which are turned to one another by a fixed angle in order to achieve a two-dimensional deflection. By varying the written grating period, the diffractive gratings can realise a locally different deflection across the surface area of the deflection unit in order to realise or at least to support the function of a field lens.
In a controllable deflection grating whose grating period is variable so to set a desired diffraction angle, there is a minimum settable period due to the spatial resolution with which the deflection grating can be controlled. If the period is set, for example, using a grid-like electrode structure, there are limitations to the width and distance of the electrodes caused by the manufacturing process. In addition, electric stray fields or diffusing or diffractive components of the deflection grating, for example, cause cross-talking among set neighbouring phase values. They can also reduce the diffraction efficiency and thus cause the occurrence disturbing diffused light or light in higher diffraction orders.
Since in a grid-shaped diffractive structure the diffraction angle is inversely proportional to the periodicity of the diffractive structure, the available angular range and thus the tracking range of a single diffraction device is limited by the producible electrode pitch.
However, to be able to watch a 3D scene comfortably at various viewing angles, a display is required to have a large tracking range at a variable observer distance. A solution is thus sought which, the limited diffraction angle of a deflection element notwithstanding, provides a tracking range which is larger than that achievable with such element.